The present invention relates generally to providing and controlling dose enhancements in high energy particle beam (electrons or photons) treatment of a target area in a patient's body. More specifically, the present invention relates to methods and systems that use a permanent magnet assembly that creates a dose enhancement at the target area along the high energy particle beam path, compared to the dose delivered to surrounding regions of the patient's body along the beam path.
A photon beam interacts with tissue in a well-understood manner. Photons themselves, whatever their energy, do not produce ionization (e.g., damage). Rather, the photons interact with the electrons and nuclei of tissue constituents. This interaction results mostly in electrons (and positrons) receiving a substantive amount of energy from the photon beam. These moving charged particles are responsible for the ionization that damages the tissue.
The angular distribution of the electrons initially depends on the energy of the photons, but scattering interactions lead to more photons, that then result in more electrons, in what is called a “cascade,” that generates a path of ionizing particles that becomes broader and the process becomes diffusive. These effects have two results that impact the effectiveness of radiation treatment with photon beams. First, the initial photon beam target area is not well defined, being mostly dependent on the density of the tissue in its path, with a component due to attenuation, i.e., less beam is left the deeper it penetrates into the subject. Second, the cascade further results in dose delivery downstream to the target area, as well as outside the beam boundary.
Similar considerations apply to electron beams used in radiation treatment. As the electron travels through tissue, it produces ionization along its entire path, losing small amounts of its energy to each ionization. It can also undergo scattering with an electron (or nucleus) in the tissue, and this scattering transfers a large fraction of its energy. Once an electron interacts with the tissue in this manner, a cascade of electrons and photons is produced, which, qualitatively, is not different from that produced by a photon.
Radiation therapy planning is a mature, yet still evolving practice. During radiation therapy, an operator will typically manipulate a beam profile and/or beam directions to attempt to maximize the radiation dosage to the target area(s), while minimizing the amount of radiation dosage to the surrounding and non-adjacent tissue in the patient body.
The actual path of photon beams cannot be affected in a measurable manner. On the other hand, the path of charged particles is affected by electric and magnetic fields, a phenomenon used in CRTs, for example. For practical reasons, magnetic fields are generally preferable for this purpose. For any given magnetic (or electric) field strength the effect on the path of the beam will be lower for higher particle energy, thus the beam of electrons will be much less affected than the cascade electrons. A moving charged particle in a magnetic field will see a force perpendicular to its direction of motion and perpendicular to the magnetic field vector, so that it will tend to circle the magnetic field line. This results in a corkscrew motion where speed is preserved, but the velocity vector has a component along the field lines and another component around the field lines. In the absence of impediments, the electrons will travel along magnetic field lines and diffuse towards the region with the weakest magnetic field and away from the strong magnetic field region.
To that end, Whitmire et al., “Magnetic modification of the electron-dose distribution in tissue and lung phantoms,” Med. Phys 5(5), September/October 1978 (the complete disclosure of which is incorporated by reference) describes the use of an electromagnet to generate a moderately strong transverse-magnetic field to modify electron-dose distributions in tissue, and discusses the use of superconducting magnets for the same purpose. Lee and Ma, “Monte Carlo characterization of clinical electron beams in transverse magnetic fields,” Phys. Med. Biol. 45(10):2947-2967, 2000 (the complete disclosure of which is incorporated by reference) studied the characteristics of the electron beam of a clinical linear accelerator in the presence of 1.5 and 3.0 Tesla transverse magnetic fields to assess the possibility of using magnetic fields in conjunction with modulated electron radiation therapy. Longitudinal magnetic fields have also been found to enhance the depth of dosage distribution of an electron beam when the field was applied prior to the beam reaching the target. (Earl and Ma, Med. Phys. 29(4):484-491, 2002, the complete disclosure of which is incorporated by reference).
Similarly, U.S. Pat. No. 5,974,112 to Reiffel, the complete disclosure of which is incorporated herein by reference, describes a method of controlling and enhancing dose in the target area of a patient's body by using a topical magnet in the form of an array of magnet coils to create a magnetic field within a subject undergoing radiation therapy. As described in Reiffel, the magnet is characterized as producing a “magnetic field configuration having a magnetic field component across the beam path and having a magnetic field gradient component along the beam path which cause the dose enhancement, the dose enhancement being changeable during beam use by changing the magnetic field configuration during beam use, wherein the topical magnetic field can be produced by an array of magnet coils.” In one particular embodiment, Reiffel suggests using superconducting magnet coils. In a separate study, Reiffel et al. examined the effects of a small super conducting magnet on the control over photon dose effects. (Phys. Med. Biol. 45:N177-N182, 2000, the complete contents of which is incorporated herein). In water phantoms, Reiffel et al. found that the effects of a locally strong transverse magnetic field with large gradients extended to 3 to 4 cm or more beyond the warm face of the cryostat.
Currents in superconductors and normal conductors may be used to produce magnetic fields of distributions calculable from well known physical principles. In practice, however, both normal conductors and superconductor coils are impractical for enhancing dosage in a photon and electron beam treatment to a target area in a patient's body.
Conventional resistive coil magnets generate a large amount of heat from the currents used to create the magnetic field. The heat is typically controlled by using thicker conductors for the coils (e.g., less resistance) and using active water cooling. For high magnetic fields, the magnetic coils are very large and cumbersome and electrical consumption is high. Because of the cumbersome nature and need for active cooling, the coils are difficult to reposition and reorient. Furthermore, they are large and the coil magnets need to be positioned relatively far away from the patient, thus limiting its ability to create sufficient magnetic fields within the patient's body (the field strength drops rapidly with distance from the magnet).
While the use of superconducting coil magnets eliminate the heat dissipation problem, the superconducting coils have the additional requirement of needing to be kept cold, typically between about 4 Kelvin and 10 Kelvin. Such cooling typically requires a liquid helium circulating system or active cryogenic coolers (operated by electricity, and also generating heat that needs to be dissipated). The whole assembly has to be kept within a vacuum cryostat. Typically, between the superconducting coil and the outside vacuum cryostat, there is an intermediate temperature shield (40 to 77 Kelvin) kept cold by liquid nitrogen or an active cryogenic cooler. Some cooling systems are dry (only an electric cooler), some are liquid (helium and nitrogen), and others hybrid, and use the active cooler to preserve the cryogenic liquids. Typically, multiple layers of superinsulation (aluminized Mylar) are placed between the superconducting coils and the shield, and between the shield and the vacuum cryostat. Care must be taken that the superinsulation does not touch the cryostat and provide heat conducting paths (called heat shorts). Consequently, the superconducting coil magnet assembly is a bulky system, and one that needs to be fabricated with great care and maintained both at vacuum and low temperature continuously, whether in use or not. A particular disadvantage is the space introduced between the magnetic field-generating coils and the outside of the cryostat, this space taken up by two superinsulation blankets, the intermediate shield, and the low temperature container, the intermediate temperature container and the room temperature container. This space reduces the magnetic field strength at the patient.
In another aspect of this practice, for superconducting magnets, considerations of restrictions on winding of the wire and the need for cooling and insulation, restrict the practically achievable magnetic field configurations. The winding of superconducting coils for achieving high fields is an art. Winding configurations are limited by the need to avoid sharp bends and to minimize stresses on the wire. The stresses arise from the interaction of the current in the coil with the magnetic field the current creates. When winding shapes depart from the perfectly circular, even if they are elliptical, these problems start to exacerbate. Therefore, for practical purposes, most magnets are typically limited to simple circular confirmations.
The topical superconducting magnets, because of winding, cooling and insulation constraints, if unobtrusive, typically lend themselves only to the creation of non-uniform fields, all with the basic topology of a dipole. While proven useful, these magnetic fields are less than optimal.
From the above, it is apparent that improvements over the array of coil magnet are needed. In particular, what are needed are low cost, simple systems and methods which enhance a radiation dose to a target area in comparison to the radiation dose to the surrounding regions in the patient's body. Preferably, the methods and systems should be robust and easily reconfigurable.